A port number is just a number, so you would transmit it just like you would any other data. Of course, you would need to transmit it before you stop using the old port because you wouldn't be able to transmit anything to the client otherwise. I think you need to be more specific about your issue. Why do you think that transmitting this data is different to any other? Are you saying, without actually saying, that you have already stopped using the old port? In that case you can't transmit anything. You'd have to send the port number through some other means, e.g. email or a web site, and have the user update the app manually.
